Disallow replacing joins with scans in problematic cases.

Commit e7cb7ee14, which introduced the infrastructure for FDWs and
custom scan providers to replace joins with scans, failed to add support
handling of pseudoconstant quals assigned to replaced joins in
createplan.c, leading to an incorrect plan without a gating Result node
when postgres_fdw replaced a join with such a qual.

To fix, we could add the support by 1) modifying the ForeignPath and
CustomPath structs to store the list of RestrictInfo nodes to apply to
the join, as in JoinPaths, if they represent foreign and custom scans
replacing a join with a scan, and by 2) modifying create_scan_plan() in
createplan.c to use that list in that case, instead of the
baserestrictinfo list, to get pseudoconstant quals assigned to the join;
but #1 would cause an ABI break.  So fix by modifying the infrastructure
to just disallow replacing joins with such quals.

Back-patch to all supported branches.

Reported by Nishant Sharma.  Patch by me, reviewed by Nishant Sharma and
Richard Guo.

Discussion: https://postgr.es/m/CADrsxdbcN1vejBaf8a%2BQhrZY5PXL-04mCd4GDu6qm6FigDZd6Q%40mail.gmail.com

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index d218be50216a388a66c1973bfaff59503b96d82d..22f900e3d107ef495052aafece23beb54392face 100644 (file)
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -2188,6 +2188,32 @@ SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM
    1
 (10 rows)
 
+-- join with pseudoconstant quals, not pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1 AND CURRENT_USER = SESSION_USER) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
+                                  QUERY PLAN                                   
+-------------------------------------------------------------------------------
+ Limit
+   Output: t1.c1, t2.c1, t1.c3
+   ->  Sort
+         Output: t1.c1, t2.c1, t1.c3
+         Sort Key: t1.c3, t1.c1
+         ->  Result
+               Output: t1.c1, t2.c1, t1.c3
+               One-Time Filter: (CURRENT_USER = SESSION_USER)
+               ->  Hash Join
+                     Output: t1.c1, t1.c3, t2.c1
+                     Hash Cond: (t2.c1 = t1.c1)
+                     ->  Foreign Scan on public.ft2 t2
+                           Output: t2.c1
+                           Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
+                     ->  Hash
+                           Output: t1.c1, t1.c3
+                           ->  Foreign Scan on public.ft1 t1
+                                 Output: t1.c1, t1.c3
+                                 Remote SQL: SELECT "C 1", c3 FROM "S 1"."T 1"
+(19 rows)
+
 -- non-Var items in targetlist of the nullable rel of a join preventing
 -- push-down in some cases
 -- unable to push {ft1, ft2}

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 73b839efe4719d43550845332857273b2d87041e..1baf19768a5a7188f7f173935e81c8f2d8fda0e3 100644 (file)
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -583,6 +583,9 @@ SELECT t1c1, avg(t1c1 + t2c1) FROM (SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM ft1 t2, ft2 t3 WHERE t2.c1 = t3.c1 AND t2.c2 = t1.c2) q ORDER BY t1."C 1" OFFSET 10 LIMIT 10;
 SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM ft1 t2, ft2 t3 WHERE t2.c1 = t3.c1 AND t2.c2 = t1.c2) q ORDER BY t1."C 1" OFFSET 10 LIMIT 10;
+-- join with pseudoconstant quals, not pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1 AND CURRENT_USER = SESSION_USER) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
 
 -- non-Var items in targetlist of the nullable rel of a join preventing
 -- push-down in some cases

diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index 27614d614b929a99f5a2b450747312196caa5320..d6786613216a51954660eca1641c7ba54c56db6a 100644 (file)
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -22,6 +22,7 @@
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "optimizer/planmain.h"
+#include "optimizer/restrictinfo.h"
 
 /* Hook for plugins to get control in add_paths_to_joinrel() */
 set_join_pathlist_hook_type set_join_pathlist_hook = NULL;
@@ -124,6 +125,7 @@ add_paths_to_joinrel(PlannerInfo *root,
 {
    JoinPathExtraData extra;
    bool        mergejoin_allowed = true;
+   bool        consider_join_pushdown = false;
    ListCell   *lc;
    Relids      joinrelids;
 
@@ -307,13 +309,25 @@ add_paths_to_joinrel(PlannerInfo *root,
        hash_inner_and_outer(root, joinrel, outerrel, innerrel,
                             jointype, &extra);
 
+   /*
+    * createplan.c does not currently support handling of pseudoconstant
+    * clauses assigned to joins pushed down by extensions; check if the
+    * restrictlist has such clauses, and if so, disallow pushing down joins.
+    */
+   if ((joinrel->fdwroutine &&
+        joinrel->fdwroutine->GetForeignJoinPaths) ||
+       set_join_pathlist_hook)
+       consider_join_pushdown = !has_pseudoconstant_clauses(root,
+                                                            restrictlist);
+
    /*
     * 5. If inner and outer relations are foreign tables (or joins) belonging
     * to the same server and assigned to the same user to check access
     * permissions as, give the FDW a chance to push down joins.
     */
    if (joinrel->fdwroutine &&
-       joinrel->fdwroutine->GetForeignJoinPaths)
+       joinrel->fdwroutine->GetForeignJoinPaths &&
+       consider_join_pushdown)
        joinrel->fdwroutine->GetForeignJoinPaths(root, joinrel,
                                                 outerrel, innerrel,
                                                 jointype, &extra);
@@ -321,7 +335,8 @@ add_paths_to_joinrel(PlannerInfo *root,
    /*
     * 6. Finally, give extensions a chance to manipulate the path list.
     */
-   if (set_join_pathlist_hook)
+   if (set_join_pathlist_hook &&
+       consider_join_pushdown)
        set_join_pathlist_hook(root, joinrel, outerrel, innerrel,
                               jointype, &extra);
 }

diff --git a/src/backend/optimizer/util/restrictinfo.c b/src/backend/optimizer/util/restrictinfo.c
index 8a2b6ada52d3edb76e866b184b409047556de684..47e3084209c39dc01ca705d2b7b421a6d288b83a 100644 (file)
--- a/src/backend/optimizer/util/restrictinfo.c
+++ b/src/backend/optimizer/util/restrictinfo.c
@@ -509,6 +509,35 @@ extract_actual_join_clauses(List *restrictinfo_list,
    }
 }
 
+/*
+ * has_pseudoconstant_clauses
+ *
+ * Returns true if 'restrictinfo_list' includes pseudoconstant clauses.
+ *
+ * This is used when we determine whether to allow extensions to consider
+ * pushing down joins in add_paths_to_joinrel().
+ */
+bool
+has_pseudoconstant_clauses(PlannerInfo *root,
+                          List *restrictinfo_list)
+{
+   ListCell   *l;
+
+   /* No need to look if we know there are no pseudoconstants */
+   if (!root->hasPseudoConstantQuals)
+       return false;
+
+   /* See if there are pseudoconstants in the RestrictInfo list */
+   foreach(l, restrictinfo_list)
+   {
+       RestrictInfo *rinfo = lfirst_node(RestrictInfo, l);
+
+       if (rinfo->pseudoconstant)
+           return true;
+   }
+   return false;
+}
+
 
 /*
  * join_clause_is_movable_to

diff --git a/src/include/optimizer/restrictinfo.h b/src/include/optimizer/restrictinfo.h
index e0e1ade425ef5e374bf9f780cc8286dbdc777494..b24ff727b0ef9b68bd0ebdb2c5960abb78df2da4 100644 (file)
--- a/src/include/optimizer/restrictinfo.h
+++ b/src/include/optimizer/restrictinfo.h
@@ -49,6 +49,8 @@ extern void extract_actual_join_clauses(List *restrictinfo_list,
                                        Relids joinrelids,
                                        List **joinquals,
                                        List **otherquals);
+extern bool has_pseudoconstant_clauses(PlannerInfo *root,
+                                      List *restrictinfo_list);
 extern bool join_clause_is_movable_to(RestrictInfo *rinfo, RelOptInfo *baserel);
 extern bool join_clause_is_movable_into(RestrictInfo *rinfo,
                                        Relids currentrelids,